This invention relates to thermo-magneto-optic recording elements and, more particularly to a method of preparing thermo-magneto-optic elements.
As used herein, the term "thermo-magneto-optic (TMO) recording element" refers to a multi-layer structure used in the thermo-magneto-optic write/read process for selectively modifying the characteristic of an incident polarized light beam in accordance with a magnetization pattern formed in the TMO recording layer which is one of the layers of the recording element. Typically such recording elements include a substrate, a thermo-magneto-optic recording layer and one or more corrosion-preventing and/or anti-reflection layers of a dielectric or semi-conductive material. When the TMO layer is a rare-earth transition metal layer, a corrosion-preventing layer is required on either side of the TMO layer in order to prevent deterioration and ultimate destruction of the magneto-optic properties of the TMO layer. It is desirable that this corrosion-preventing layer also serve as an anti-reflection layer.
A preliminary function of the anti-reflection layer in a TMO recording element is to enhance the rotation angle .THETA..sub.K caused by the Kerr effect (in reflective systems) and Faraday effect (in transmission systems). This enhancement is of importance because the degree of rotation of a polarized collimated beam of light capable of being read out of a TMO device is extremely small, in most cases, less than a fraction of a degree. Thus, the signal to noise ratio of the readout signal requires enhancement. As indicated above, it is desired that the anti-reflective layer serves the dual function of enhancing the rotation angle and also preventing corrosion of the TMO layer. Thus, an anti-reflection corrosion preventing dielectric or semi-conductive material layer is required between the substrate and the rare-earth transition metal TMO layer. Such structures are described in an article published in the Japanese Journal of Applied Electronics, Vol. 8, No. 2, page 93-96 entitled "Study on High Reliability of a Magneto-Optic Medium With A Multi-Layer Structure" authored by K. Ohta et al. (1984) and in U.S. Pat. No. 4,719,154 by T. K. Hatwar which is assigned to the same assignee as this immediate application.
A problem that exists in a structure having a corrosion prevention, anti-reflection layer between a polymer substrate and the TMO layer is that upon being subjected to large changes in temperature, buckling and delamination between the polymer substrate layer and the corrosion preventing, anti-reflection layer occurs thereby either rendering the recording element inaccurate or totally inoperative. In other words, the structure consisting of the substrate and film package, must survive temperature excursions from approximately +60.degree. C. to -40.degree. C. in order to withstand shipping and use conditions. It is believed that the delamination or buckling that occurs between the substrate and the adjacent corrosion resistant, anti-reflection layer is due to stresses in the layers caused by the different thermal expansion coefficients of these two adjacent layers. Such deformities cannot be tolerated in a thermo-magneto-optic recording element because of the very small changes in rotation of the collimated polarized light beam used in the readout operation.